Scientists said they have documented for the first time that reptiles, like people, experience rapid eye movement (REM) sleep and another sleep stage called slow-wave sleep.

Until now, only mammals and birds were known to experience these.

Because REM sleep is when dreaming occurs in people, the findings suggest that these lizards dream, too. But, what would bearded dragons dream about?

"If you forced me to speculate and to use a loose definition of dreaming, I'd speculate that those dreams are about recent notable events: insects, maybe a place where there are good insects, an aggressive male in the next terrarium, et cetera," said neuroscientist Gilles Laurent, director of the Max Planck Institute for Brain Research in Germany.

"If I were an Australian dragon living in Frankfurt, I'd be dreaming of a warm day in the sun."

When REM and slow-wave sleep first evolved has remained a mystery.

The discovery that reptiles share these important sleep stages with mammals and birds, sister groups among the land vertebrates, suggests the sleep traits emerged far earlier than previously suspected in the common evolutionary ancestors of the three groups.

Slow-wave sleep, also called deep sleep, is the most restful sleep period, marked by slow brainwaves and little dreaming.

Lizards averaged 350 80-second-long cycles

These stages are thought to be useful for consolidation, storage and erasure of memories and other purposes.

The researchers placed probes inside the brains of five bearded dragons, which can reach 60 centimetres in length, to measure electrophysiological activity during sleep.

While people experience four or five long slow-wave/REM sleep cycles nightly, the lizards averaged 350 80-second-long cycles.

Some of the telltale signs of these sleep stages, seen in the brain's hippocampus in mammals, were found in a more primitive brain region, the dorsal ventricular ridge, in the lizards.

Some scientists had hypothesised REM and slow-wave sleep might be linked to warm-bloodedness and evolved independently in birds and mammals.

Mr Laurent said the findings suggest these sleep traits probably evolved at least as long ago as in the common ancestor of reptiles, birds and mammals — small lizard-like animals that lived between 300 million and 320 million years ago.